Preheat the oven: Set your oven to 350°F (175°C). Grease a 9x13-inch baking pan thoroughly.
Prepare the brownie batter: Follow the instructions on the box to make the brownie mix. Set aside.
Mix the cake batter: Prepare the cake mix according to the directions on its box.
Layer the batters: Pour half of the cake batter into the prepared pan. Then, spoon half of the brownie batter over it.
Create a marble effect: Use a knife to gently swirl the two batters together for a beautiful, marbled look.
Bake: Place the pan in the preheated oven and bake for approximately 35 minutes, or until a toothpick inserted in the center comes out mostly clean.
Cool: Remove from the oven and let the cake cool for about 20 minutes.
Top with fudge: Warm the hot fudge topping slightly, then drizzle generously over the cooled cake.
